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BONDED vs UNBONDED POSTTENSIONING SYSTEM
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hemal
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PostPosted: Sat Nov 06, 2010 2:16 am    Post subject: BONDED vs UNBONDED POSTTENSIONING SYSTEM Reply with quote

Dear Sefi Members,

I have prepared two power point presentation, one one "BONDED vs UNBONDED POSTTENSIONING SYSTEM" and other on "TYPES OF PT BEAMS BASED ON FLEXURAL FAILURE". Plz find it here as attachment (open as READ-ONLY). References are also mentioned in presentation.

After referring the attached presentations can we say that "BONDED PT STRUCTURES ARE MORE DUCTILE THAN UNBONDED STRUCTURES AND SHALL BE PROFFERED IN SEISMIC AREA?

Regards

Hemal Mistry
Surat

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arunsefi
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PostPosted: Sun Nov 14, 2010 10:02 am    Post subject: Reply with quote

Dear Mr. Hemal Mistry,
Thanks for providing your views on sefi. I do not understand on what basis you have arrived to such statement:

"After referring the attached presentations can we say that "BONDED PT STRUCTURES ARE MORE DUCTILE THAN UNBONDED STRUCTURES AND SHALL BE PROFFERED IN SEISMIC AREA? "

This statement tells that you have just touched the tip of an iceburg. Not even gone through its mordern design criteria of ACI and other renowned literatures as well as codes and standards. Books are not updated with the recent advances. Even our Indian Code is not updated since 1980.

PT Design is not only just a design of Tendos. It covers all the parameters necessary and mendatory for stability, sustainability and durability of a basic structure. A PT member designed with unbonded tendons can possess equal or more ductility than bonded PT system. This depends upon its design. There will allways be sufficient conventional reinforcement along with tendons to take care of ductility and other parameters.  

You will find enough literature on internet for this kind of study. Anyways let me get you few references for your persual. These are radily available on net:

1. An ACI paper 68-13"Comparative Study of Prestressed Concrete Beams, with and without bond" by Alan Mattock, Jun Yamazaki and Basil Kattula. They did experimental research on it and found that the ductility and strength as well as serveaciility characteristics of the beams tested having unbonded tendons found better than bonded post-tensioned beams. (Refer page number 125 for the conclusion).

2. there is another article published by VSL (VSL report series 4.2: post-tensioned slabs) awailable on website for free. Please refer page 3 and 4 of the same article. VSL is known as gient group internationally. He is expert in PT design. Lots of experience and challenging projects. They must be partial about their bonded system, still they have writtern that both the systems works satisfactorily when designed properly.

PT does not have any direct relevance with seismic forces. PT is meant for permanent loadings where as the seismic forces are transient in nature. They shall not been designed with PT. One must DETAIL the conventional reinforcement to ensure ductility as well as resistance against seismic forces. There is no point remains whether PT is unbonded or "grouted". I have intentionally used the word "grouted" here.

Kindly let me have your views on it.

Thanks.

Arun
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hemal
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PostPosted: Mon Nov 15, 2010 4:03 am    Post subject: BONDED vs UNBONDED POSTTENSIONING SYSTEM Reply with quote

Dear Mr. Arun,

Thanx for your comments. I was waiting for comments and discussion on my presentations since long.

I would like to give some more comments:

(1) I agree with you that ACI code is advanced for PT. It has some provisions to limit reinforcement ratio (both PT & Rebar) to ensure Under-reinforced, ductile sections. It also suggests some minimum bonded steel for UNBONDED tendons.

(2) It is very difficult to estimate stress/strain in UNBONDED tendons. and stresses in tendon at failure may be much more less than the estimated one. IS:1343 suggests rigorous analysis or filed test to determine stresses in UNBONDED tendons. Which is hardly carried out. Even most of the softwares utilize some equations suggested by ACI code to estimate the stress in UNBONDED tendons while designing with indian code.

(3) My comments on DUCTILITY was from basic definition of DUCTILE FAILURE (amount and location of steel shall be such that failure is due to steel reaching its tensile strength before concrete reaches its compressive strength). I have referred the reference "ACI paper 68-13", where a simply supported and 2 span continuous beams (bonded and unbonded) were tested and compared. They found that UNBONDED beam behaves as flexural member rather than shallow tied arch. (it means i think there is some bond between concrete and tendon and tendon is not free as assumed in basics). However, This tests were carried out for particular reinforcement ratio (here low) and depth of neutral axis. I think for diffrent values of reinforcement ratio (here low) and depth of neutral axis tests shall be carried out, to arrive at a conclusion.

(4) COMMENT FROM MR. ARUN : "PT does not have any direct relevance with seismic forces. PT is meant for permanent loadings where as the seismic forces are transient in nature. They shall not been designed with PT. One must DETAIL the conventional reinforcement to ensure ductility as well as resistance against seismic forces. There is no point remains whether PT is unbonded or "grouted". I have intentionally used the word "grouted" here".

We do not have separate seismic provisions/codes for PT structures. Generally seismic+gravity moments will be much more higher than gravity moments. If u utilize tendons for gravity moments and provide conventional reinforcement for seismic moments, amount of conventional reinforcement will be very high, which may lead to over-reinforced sections with tensile/compressive stresses higher than permissible stresses assumed in PT design. Many times we may need to increase tendons or section sizes to limit reinforcement ratio, stresses, deflections etc. That means you can not separate Gravity load design with tendons & seismic design with reinforcement.

Many times PT structures are designed by PT contractors for gravity loads only (however this also shall be verified by main structural consultant).and seismic design is left for main structural consultant. Main structural consultant should note that by just providing conventional reinforcement for seismic moments will not be sufficient. He/she also needs to check limiting reinforcement ratio to ensure ductility, stresses, deflections (cracked or uncracked based on value of moments) etc. for seismic loads combined with gravity loads.

Regards

Hemal Mistry
Surat

--- On Sun, 14/11/10, arunsefi <forum@sefindia.org> wrote:
Quote:

From: arunsefi <forum@sefindia.org>
Subject: [SEFI] Re: BONDED vs UNBONDED POSTTENSIONING SYSTEM
To: general@sefindia.org
Date: Sunday, 14 November, 2010, 4:02 AM

Dear Mr. Hemal Mistry,
Thanks for providing your views on sefi. I do not understand on what basis you have arrived to such statement:

"After referring the attached presentations can we say that "BONDED PT STRUCTURES ARE MORE DUCTILE THAN UNBONDED STRUCTURES AND SHALL BE PROFFERED IN SEISMIC AREA? "

This statement tells that you have just touched the tip of an iceburg. Not even gone through its mordern design criteria of ACI and other renowned literatures as well as codes and standards. Books are not updated with the recent advances. Even our Indian Code is not updated since 1980.

PT Design is not only just a design of Tendos. It covers all the parameters necessary and mendatory for stability, sustainability and durability of a basic structure. A PT member designed with unbonded tendons can possess equal or more ductility than bonded PT system. This depends upon its design. There will allways be sufficient conventional reinforcement along with tendons to take care of ductility and other parameters.

You will find enough literature on internet for this kind of study. Anyways let me get you few references for your persual. These are radily available on net:

1. An ACI paper 68-13"Comparative Study of Prestressed Concrete Beams, with and without bond" by Alan Mattock, Jun Yamazaki and Basil Kattula. They did experimental research on it and found that the ductility and strength as well as serveaciility characteristics of the beams tested having unbonded tendons found better than bonded post-tensioned beams. (Refer page number 125 for the conclusion).

2. there is another article published by VSL (VSL report series 4.2: post-tensioned slabs) available on website for free. Please refer page 3 and 4 of the same article. VSL is known as gient group internationally. He is expert in PT design. Lots of experience and challenging projects. They must be partial about their bonded system, still they have written that both the systems works satisfactorily when designed properly.

PT does not have any direct relevance with seismic forces. PT is meant for permanent loadings where as the seismic forces are transient in nature. They shall not been designed with PT. One must DETAIL the conventional reinforcement to ensure ductility as well as resistance against seismic forces. There is no point remains whether PT is unbonded or "grouted". I have intentionally used the word "grouted" here.

Kindly let me have your views on it.

Thanks.

Arun
     



     


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arunsefi
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PostPosted: Mon Nov 15, 2010 3:15 pm    Post subject: Reply with quote

Dear Mr. Hemal,

Thanks for your response. Let me discuss one by one as below;


(1) Comment from Mr. Hemal:  I agree with you that ACI code is advanced for PT. It has some provisions to limit reinforcement ratio (both PT & Rebar) to ensure Under-reinforced, ductile sections. It also suggests some minimum bonded steel for UNBONDED tendons.


Arun: That is what I wanted to convey. It has got almost all stipulations necessary to achieve a good design. Most of the researches on PT has been carried out in America and published in ACI journals. Also it is updated almost every year. I always recommend following no other code but ACI for analysis and design of PT Floor.

(2) Comment from Mr. Hemal:  It is very difficult to estimate stress/strain in UNBONDED tendons. and stresses in tendon at failure may be much more less than the estimated one. IS:1343 suggests rigorous analysis or filed test to determine stresses in UNBONDED tendons. Which is hardly carried out. Even most of the softwares utilize some equations suggested by ACI code to estimate the stress in UNBONDED tendons while designing with indian code.

Arun: You are correct to some extent; however there have been certain researches already carried out and found experimentally that whatever formulations prescribed in the ACI code are such that they results conservative.

IS : 1343 is not updated and it is more for design of PT girders. There is hardly any recommendation for PT Flat Slab design in it. There is one recommendation regarding PT Flat slab design in IS:1343 : clause 21.1.2 Analysis of slabs spanning in two directions at right angles in general, the provisions of IS:456 shall apply.  What should one do in this case?



On the contrary if you go through ACI 318 code you will find separate chapters for RCC Slabs and PT. In chapter 18 they have clearly mentioned not to use mostly all recommendations prescribed for RCC slabs while you are designing PT slab.


(3) Comment from Mr. Hemal:  My comments on DUCTILITY was from basic definition of DUCTILE FAILURE (amount and location of steel shall be such that failure is due to steel reaching its tensile strength before concrete reaches its compressive strength). I have referred the reference "ACI paper 68-13", where a simply supported and 2 span continuous beams (bonded and unbonded) were tested and compared. They found that UNBONDED beam behaves as flexural member rather than shallow tied arch. (it means i think there is some bond between concrete and tendon and tendon is not free as assumed in basics). However, This tests were carried out for particular reinforcement ratio (here low) and depth of neutral axis. I think for diffrent values of reinforcement ratio (here low) and depth of neutral axis tests shall be carried out, to arrive at a conclusion.

Arun: Reinforcement ratio is one of the parameters to check during the design itself. Generally PT member has lesser depth but wider section. The concrete section is enough to resist the designed compression stresses. PT does induce direct compressive stress in section; additionally it induces bending stresses which is opposite to the bending stress due to applied loading. Say for example at mid-span of the beam at top fiber, there would be direct pre-compression stresses as well as compression stress due to applied loading but there will be tensile stresses achieved due to cable profile. Hence during design itself, this thing shall be taken care.
And these thing will be similar for both kind of system either it is unbonded or grouted.


(4) Comment from Mr. Hemal:  We do not have separate seismic provisions/codes for PT structures. Generally seismic+gravity moments will be much more higher than gravity moments. If u utilize tendons for gravity moments and provide conventional reinforcement for seismic moments, amount of conventional reinforcement will be very high, which may lead to over-reinforced sections with tensile/compressive stresses higher than permissible stresses assumed in PT design. Many times we may need to increase tendons or section sizes to limit reinforcement ratio, stresses, deflections etc. That means you can not separate Gravity load design with tendons & seismic design with reinforcement.
Many times PT structures are designed by PT contractors for gravity loads only (however this also shall be verified by main structural consultant).and seismic design is left for main structural consultant. Main structural consultant should note that by just providing conventional reinforcement for seismic moments will not be sufficient. He/she also needs to check limiting reinforcement ratio to ensure ductility, stresses, deflections (cracked or uncracked based on value of moments) etc. for seismic loads combined with gravity loads.

Arun: There are several standards available such that IBC, UBC which is having international platform for design.

It is not always that the governing critical load combination has the seismic component. This may be true for short spanned beams. If we think about larger spans, it may be critical in Gravity load combination even. Normally PT is applied to larger spans. Let me explain what might be the design procedure in our scenario:


  1. PT Elements to be designed as per the structural configuration decided by the structural consultant and PT designer mutually.
  2. Designed sections of PT elements shall be sent to structural consultant for the geometry creation in 3D analysis for lateral cases.
  3. Structural consultant shall send their critical moments with its load combination to PT designer.
  4. PT designer shall provide the additional reinforcement accordingly with all related checks like limiting reinforcement ratio and compressive stresses induced in the section due to additional amount of moment. If it suffice all the requirements, design is good. Go ahead! Otherwise it needs to revise the section of PT member and entire process gets repeated.
    Wishing this must get the overview about the discussion I would be glad to receive your comments again.  

    Regards
    Arun
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ajit_g_g
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PostPosted: Wed Nov 17, 2010 8:12 am    Post subject: seismic moment in pt design Reply with quote

dear sir

We have portal frame of 4.5x15.5m bays we are going for pt beams of 600x600mm and cast  in situ slab i have a doubt regarding column moments to be considred in design whether I have to design  as per frame analysis or for only live load moment +direct (dead+live) load since you are doing pt  no deflection /momnt  is going to be created by self weight whether this logic is correct  the same condition should apply for seismic moment  what are other forces to be considred for column design

experts pl clarify my doubts

Thnks

ajitgg


If u utilize tendons for gravity moments and provide conventional reinforcement for seismic moments, amount of conventional reinforcement will be very high, which may lead to over-reinforced sections with tensile/compressive stresses higher than permissible stresses assumed in PT design. Many times we may need to increase tendons or section sizes to limit reinforcement ratio
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hemal
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PostPosted: Fri Nov 19, 2010 5:49 am    Post subject: BONDED vs UNBONDED POSTTENSIONING SYSTEM Reply with quote

Dear ajitgg,

In post-tensioned structures DL and part/full LL is balanced by tendon forces, but it shall not be assumed that balanced force will not be transferred to columns. DL, LL are balanced but columns are subjected to tendon forces of different stages (transfer,service,construction stages etc) and different amount due to different losses. Actually this is a case of STAGGED ANALYSIS. In structure model tendon(PT) forces shall be applied as load for transfer & service stages. If tendons are not stressed at a time, it shall be defined as different load case (e.g. in a high rise building, tendons at different floors are stressed at different time with different DL,LL at each stage). All tendon forces shall be defined properly with proper stages and shall be combined as per codal provision with EQ, WIND, DL,LL etc. loads. Columns/beams shall be designed for forces, which are result of STAGGED ANALYSIS with load combinations as mentioned above.

Most of the PT design softwares, uses substitute frame analysis, which may be valid for Beams/slabs and not for columns. They do not have facility to model & analyze whole structure.

Softwares like SAP2000, STAAD, STRAP etc have facility to input tendon forces. But may not have facility to define it for different time and stages. Also we need to model different tendon forces & profile for all beams/slabs in a structure, which is time consuming.

As per my knowledge, STRAP have inbuilt PT DESIGN MODULE. In this software you can model whole structure. U can analyze structure for Gravity,EQ,Wind etc forces. Then PT beam/slab can be designed with different stages. PT (tendon) forces can be combined with other loads. Structure can be again analyzed for design of columns.

Hope this helps.

Regards
Hemal Mistry
Surat



--- On Wed, 17/11/10, ajit_g_g <forum@sefindia.org> wrote:
Quote:

From: ajit_g_g <forum@sefindia.org>
Subject: [SEFI] Re: BONDED vs UNBONDED POSTTENSIONING SYSTEM
To: general@sefindia.org
Date: Wednesday, 17 November, 2010, 2:12 AM

dear sir

We have portal frame of 4.5x15.5m bays we are going for pt beams of 600x600mm and cast in situ slab i have a doubt regarding column moments to be considred in design whether I have to design as per frame analysis or for only live load moment +direct (dead+live) load since you are doing pt no deflection /momnt is going to be created by self weight whether this logic is correct the same condition should apply for seismic moment what are other forces to be considred for column design

experts pl clarify my doubts

Thnks

ajitgg


If u utilize tendons for gravity moments and provide conventional reinforcement for seismic moments, amount of conventional reinforcement will be very high, which may lead to over-reinforced sections with tensile/compressive stresses higher than permissible stresses assumed in PT design. Many times we may need to increase tendons or section sizes to limit reinforcement ratio
     



     


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arunsefi
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PostPosted: Tue Nov 23, 2010 1:06 pm    Post subject: Re: seismic moment in pt design Reply with quote

Dear Ajit g g,

You need to consider the net moments transferred to column due to self weight, SDL, LL and Hyperstatic Moment induced due to PT in case of strength combination.
PT balances certain load and that gets transferred to columns ultimately. You can have various load combinations according to the stages in PT.

You can ask to the PT designer for the column moment values to consider for further calculation of columns under gravity case.

REgards,

arun


ajit_g_g wrote:
dear sir

We have portal frame of 4.5x15.5m bays we are going for pt beams of 600x600mm and cast  in situ slab i have a doubt regarding column moments to be considred in design whether I have to design  as per frame analysis or for only live load moment +direct (dead+live) load since you are doing pt  no deflection /momnt  is going to be created by self weight whether this logic is correct  the same condition should apply for seismic moment  what are other forces to be considred for column design

experts pl clarify my doubts

Thnks

ajitgg


If u utilize tendons for gravity moments and provide conventional reinforcement for seismic moments, amount of conventional reinforcement will be very high, which may lead to over-reinforced sections with tensile/compressive stresses higher than permissible stresses assumed in PT design. Many times we may need to increase tendons or section sizes to limit reinforcement ratio
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er.vikramjain
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PostPosted: Wed Nov 24, 2010 6:38 am    Post subject: Reply with quote

Dear Er. Arun Sir,

which additional moment need to be consider while designing a column ? ( if it is corner column)
a.) Moment generated from PT slab?
b.) Moment generated from PT beam?
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arunsefi
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PostPosted: Fri Dec 03, 2010 12:19 pm    Post subject: BONDED vs UNBONDED POSTTENSIONING SYSTEM Reply with quote

If the slab is supported by the Beams and beams are transferring the load to column. You can consider the moment generated from PT beam. The forces and moment from PT slab will reach to the columns through the beams in that case.

You need to trace the load path first so that everything would be easier.

Thanks.
Arun


On Wed, Nov 24, 2010 at 12:08 PM, er.vikramjain <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Dear Er. Arun Sir,

which additional moment need to be consider while designing a column ? ( if it is corner column)
a.) Moment generated from PT slab?
b.) Moment generated from PT beam?







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er.vikramjain
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PostPosted: Sat Dec 04, 2010 8:40 am    Post subject: Re: BONDED vs UNBONDED POSTTENSIONING SYSTEM Reply with quote

Thank you Er. Arun Sir.

arunsefi wrote:
If the slab is supported by the Beams and beams are transferring the load to column. You can consider the moment generated from PT beam. The forces and moment from PT slab will reach to the columns through the beams in that case.

You need to trace the load path first so that everything would be easier.

Thanks.
Arun


On Wed, Nov 24, 2010 at 12:08 PM, er.vikramjain <forum> wrote:
Quote:
  Dear Er. Arun Sir,

which additional moment need to be consider while designing a column ? ( if it is corner column)
a.) Moment generated from PT slab?
b.) Moment generated from PT beam?







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